This application is based on, and claims priority to, Japanese application number 10-117043, filed Apr. 27, 1998, and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical amplifier which reflects spontaneous emission light back into the optical amplifier to improve excitation efficiency of the optical amplifier.
2. Description of the Related Art
Wavelength division multiplexing (WDM) is being used to increase transmission capacity of optical communication systems. With WDM, a plurality of wavelengths are multiplexed together and transmitted through a single fiber.
Moreover, in an optical communication system employing WDM, an optical amplifier can be used as a repeater. The transmission capacity of the optical communication system can be increased by expanding the gain wavelength band of the optical amplifier, to thereby increase the number of wavelengths which can be multiplexed together.
An Erbium (Er) doped fiber amplifier (EDFA) is a common optical amplifier which is widely used in optical communication systems employing WDM. In a typical EDFA, an excitation wavelength in a 1.48 xcexcm band or a 0.98 xcexcm band is used, and gain is obtained in a wavelength band which includes 1.53 xcexcm to 1.56 xcexcm (hereinafter referred to as the 1.55 xcexcm band).
However, with such an EDFA, the gain wavelength band is limited to the 1.55 xcexcm band. Thus, to realize a WDM optical communication system which can provide much larger capacity as will be required in the future, it is necessary to exploit a new gain wavelength band.
There is a known technique for realizing a gain wavelength band different from the 1.55 xcexcm band in an EDFA. In this technique, a wavelength band from 1.56 xcexcm to 1.62 xcexcm (hereinafter referred to as the 1.58 xcexcm band) is used. For example, there is reported a technique in which a gain of 25 dB is realized at a wavelength band from 1.57 xcexcm to 1.61 xcexcm, while adopting an excitation wavelength in the 1.55 xcexcm band. See, J. F. Massicott et el., ELECTRONICS LETTERS, Vol. 26, No. 20, pp. 1645-1646,27th September 1990.
Further, it has been lately reported that a gain at a wavelength band from 1.56 xcexcm to 1.62 xcexcm can be obtained by utilizing a laser diode of 1.48 xcexcm band or 0.98 xcexcm band as an excitation light source. See, H. Ono et el., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 5, pp. 596-597, May 1997. This technique is advantageous in that current techniques also use a laser diode excited at the 1.48 xcexcm or the 0.98 xcexcm band, and an Erbium doped optical fiber (EDF).
The following is a brief explanation about an operational principle of 1.55 xcexcm band and 1.58 xcexcm band EDFAs. As an example of a fiber amplifier doped with rare earth element, there are considered here 1.55 xcexcm band and 1.58 xcexcm band EDFAs utilizing an excitation light source of 1.48 xcexcm band or 0.98 xcexcm band.
FIGS. 1(A) and 1(B) are diagrams showing energy levels of Erbium atom. As shown in FIG. 1(A), in a conventional 1.55 xcexcm band EDFA in which the gain is obtained between 1.53 xcexcm and 1.56 xcexcm, Erbium atom is excited by an excitation light source of 1.48 xcexcm band or 0.98 xcexcm band, and there is caused induced emission between energy levels 4I13/2 and 1I15/2. By utilizing this induced emission, there is obtained a gain at 1.55 xcexcm band.
Further, as shown in FIG. 1(B), those energy levels, which are utilized in a 1.58 xcexcm band EDFA, are also energy levels 4I13/2 and 4I15/2. For example, in a 1.58 xcexcm band EDFA, Erbium atom is excited by an excitation light source of 1.48 xcexcm band or 0.98 xcexcm band, and 1.55 xcexcm band spontaneous emission will generate and lead to amplified spontaneous emission (ASE) within a front half part of the fiber. See, H. Ono et al., TECHNICAL REPORT OF IEICE, OCS97-5(1997-05), pp. 25-30. This 1.55 xcexcm band ASE is resorbed at a rear half part of the fiber, thereby causing induced emission of 1.58 xcexcm band. In this 1.58 xcexcm band EDFA, since the cross sectional area for induced emission at 1.58 xcexcm band is smaller than that of the 1.55 xcexcm band EDFA, and since a sufficiently strong 1.55 xcexcm band ASE is to be generated, it is necessary to provide an EDF of sufficient fiber length.
However, in the aforementioned 1.58 xcexcm band optical amplifier, it is difficult to confine an excitation light or 1.55 xcexcm band ASE within the EDF efficiently, thereby resulting in a conversion efficiency which is not high.
The following is an explanation of a state of spontaneous emission which is generated within an EDF of a conventional forward excitation type 1.58 xcexcm band EDFA.
FIG. 2 is a diagram showing various lights travelling through an EDF, and FIG. 3 is an enlarged view of the EDF in FIG. 2.
As shown in FIGS. 2 and 3, an excitation light source 1A of an excitation part 1 produces excitation light Tp. Excitation light Lp is provided to EDF 2 via a WDM coupler 1B. Erbium atoms within EDF 2 are excited by excitation light Lp, so that spontaneous emission lights are generated. The spontaneous emission light generated from the Erbium atoms is composed of lights advancing in random directions, and only the lights directing into those modes, through which the lights can be propagated within EDF 2, will propagatingly advance within EDF 2. The spontaneous emission lights directing into these propagation modes will be amplified, during propagation through EDF 2 in an excited state, to become 1.55 xcexcm band ASE, and will be finally emitted from EDF 2.
Since the propagation modes should exist in both of the fore and aft directions, the 1.55 xcexcm band ASE will be emitted from both ends of EDF 2. On the other hand, those spontaneous emission lights, which do not direct into any propagation modes, will be emitted outwardly from EDF 2 via an outer surface of a cladding. In FIGS. 2 and 3, the 1.55 xcexcm band ASE in the backward propagation mode is indicated by an ASE light Lab, and the 1.55 xcexcm band ASE in the forward propagation mode is indicated by an ASE light Laf.
According to the above cited reference H. Ono et al., TECHNICAL REPORT OF IEICE, the 1.55 xcexcm band ASE, which has been generated at the front half part of EDF 2, is resorbed at the rear half of EDF 2, leading to generation of induced emission (optical amplification) at the 1.58 xcexcm band. Thus, the amount of ASE in the forward propagation mode, which is outwardly emitted from the output end of the signal light Ls, has a smaller value. However, the amount of ASE in the backward propagation mode, which is emitted outwardly from the input end of the signal light Ls, has a larger value since this mode does not contribute so much to the amplification at the 1.58 xcexcm band. Further, those spontaneous emission lights, which do not direct into any propagation modes, do not contribute to optical amplification at 1.58 xcexcm and are outwardly emitted via outer surface of cladding. As a result, a part of the energy of excitation light Lp supplied from the excitation light source 1A is wastefully consumed, thereby reducing excitation efficiency.
Meanwhile, conventional optical amplifiers aiming at improving excitation efficiency include one described in U.S. Pat. No. 5,138,483, and one described in Japanese Unexamined Patent Publication No. 3-135081. Such optical amplifiers have a configuration as shown in FIG. 4.
Referring now to FIG. 4, the optical amplifier, of which excitation efficiency has been improved, includes an excitation light reflector 4 at a rear side of EDF 2 (i.e., at an outer side of the other end opposite to an input end of excitation light Lp). Excitation light reflector 4 reflects the excitation light Lp and transmits the signal light at 1.55 xcexcm band. Via excitation light reflector 4, the excitation light Lp is reflected to make one reciprocation within EDF 2, thereby improving excitation efficiency.
Unfortunately, in an optical amplifier as illustrated in FIG. 4, only the excitation light Lp (in, for example, the 1.48 xcexcm band or the 0.98 xcexcm) is reflected by excitation light reflector 4. The 1.55 xcexcm band ASE generated
within EDF 2 will be transmitted through excitation light reflector 4. Thus, the excitation efficiency will be insufficient if the optical amplifier used to amplify signals in the 1.58 xcexcm band.
Accordingly, it is an object of the present invention to provide an apparatus in which spontaneous emission light and excitation light are effectively propagated within an active optical fiber to thereby improve excitation efficiency.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
Objects of the present invention are achieved by providing an optical amplifier which includes an active optical fiber doped with a rare earth element. An excitation light generating device generates an excitation light at a predetermined wavelength band and supplies the excitation light to the active optical fiber. A spontaneous emission light reflection device reflects spontaneous emission light generated from the rare earth element excited by the excitation light and having a wavelength band different from that of the excitation light, so that a signal light is amplified as the signal light propagates within the active optical fiber. The signal light is in a wavelength band different from the wavelength band of the excitation light and the wavelength band of the spontaneous emission light.
According to such a configuration, an excitation light (such as at a 1.48 xcexcm band or a 0.98 xcexcm band) is generated by the excitation light generating device, and supplied to the active optical fiber. Within the active optical fiber, there is generated a spontaneous emission light from the rare earth element (such as Erbium) excited by the excitation light. This spontaneous emission light is propagated within the active optical fiber so as to be amplified, and thereby cause amplified spontaneous emission (ASE) at a 1.55 xcexcm band. This 1.55 xcexcm band ASE is resorbed in the course of propagation within the active optical fiber, thereby enabling optical amplification of signal light such as at 1.58 xcexcm band. Further, upon reaching the spontaneous emission light reflection device, the ASE being propagated within the active optical fiber is reflected by the spontaneous emission light reflection device, so as to be propagated in the reverse direction within the active optical fiber. By virtue of this reflection, the possibility of resorption of the spontaneous emission light is increased, so that the amplification of signal light such as at 1.58 xcexcm band can be effected at a higher excitation efficiency.
As ati example, the spontaneous emission light reflection device may include a first reflection part for reflecting the spontaneous emission light and for transmitting the signal light at a signal light input end of the active optical fiber. Further, the spontaneous emission light reflection device may include a second reflection part for reflecting the spontaneous emission light and for transmitting the signal light at a signal light output end of the active optical fiber. In addition, the spontaneous emission light reflection device may have a characteristic to transmit the excitation light, when the spontaneous emission light reflection device is provided between an output end of the excitation light generating device and one end of the active optical fiber into which the excitation light is input.
The spontaneous emission light reflection device can include the first reflection part at the signal light input end, as described above. Thus, the signal light input end of the active optical fiber is rendered to reflect ASE directing into any backward propagation mode which are difficult to be resorbed, among spontaneous emission lights generated within the active optical fiber. As a result, the ASE can be effectively utilized for amplification of the signal light. Further, by providing the second reflection part at the signal light output end of the active optical fiber, the ASE directing into any forward propagation mode is also reflected at the signal light output end, thereby enabling more effective utilization of the ASE.
Preferably, the active optical fiber is provided with, over a predetermined region along the longitudinal direction of the active optical fiber, a spontaneous emission light reflection area having a grating for reflecting the spontaneous emission light and for transmitting the signal light, and the spontaneous emission light reflection area functions as the spontaneous emission light reflection device. Concretely, the spontaneous emission light reflection area may be provided with a first reflection region arranged near a signal light input end of the active optical fiber, or a second reflection region arranged near a signal light output end of the active optical fiber.
According to such an embodiment, the ASE is rendered to be reflected by the spontaneous emission light reflection area formed within the active optical fiber. By providing the spontaneous emission light reflection area in the described manner, loss of signal light at the spontaneous emission light reflection device as well as coupling loss between the spontaneous emission light reflection device and the active optical fiber can be reduced or eliminated. Thus, the excitation efficiency is further enhanced, while enabling reduction of the noise factor.
An excitation light reflection device can also be provided at another end opposite to the one end of the active optical fiber into which the excitation light is input, for reflecting the excitation light and for transmitting the signal light.
By such an embodiment, the excitation light propagating within the active optical fiber is reflected by the excitation light reflection device, so as to go and return within the active optical fiber. As such, the excitation efficiency of the excitation light is enhanced, thereby enabling further high-powerization of the optical amplifier.
Preferably, the active optical fiber is provided with an excitation light reflection area near another end opposite to the one end of the active optical fiber into which one end the excitation light is input. The excitation light reflection area can then have a grating for reflecting the excitation light and for transmitting the signal light. As a result, the excitation light reflection area functions as the excitation light reflection device.
Thus, the excitation light is rendered to be reflected by the excitation light reflection area within the active optical fiber, so that insertion and coupling losses due to excitation light reflection device can be reduced or eliminated, thereby enabling further high-powerization of, and noise factor reduction in, the optical amplifier.
Further, with respect to the apparatus provided with the excitation light reflection device, the excitation light generating device is advantageously provided with an excitation light entrance prevention part for preventing the reflected excitation light from entering the excitation light generating device. The excitation light entrance prevention part can be an optical isolator which transmits the generated excitation light only in one direction, or an optical circulator which transmits the generated excitation light only in a certain direction between a plurality of terminals.
By providing the excitation light generating device with the excitation light entrance prevention part, there can be avoided such a situation that the operation of excitation light generating device becomes unstable due to entrance of the reflected excitation light.
In addition, with respect to any one of the aforementioned apparatuses, the active optical fiber is preferably provided with, at a surface of cladding thereof, a spontaneous emission light reflection layer for reflecting the spontaneous emission light.
By adopting such an active optical fiber, there are also confined within the active optical fiber those lights which do not direct to any propagation modes, among spontaneous emission lights generated within the active optical fiber. Thus, the excitation efficiency can be further enhanced.
The present invention further provides an active optical fiber doped with a rare earth element, including a spontaneous emission light reflection area arranged over a predetermined region along a longitudinal direction of the active optical fiber. A grating is included in the spontaneous emission light reflection area. The grating reflects the spontaneous emission light generated from the rare earth element excited by an excitation light and having a wavelength band different from that of the excitation light, and transmits a signal light at a wavelength band different from the wavelength band of the excitation light and different from the wavelength band of the spontaneous emission light.
According to such an active optical fiber, ASE directing into any propagation modes, among the spontaneous emission lights generated within the active optical fiber, can be reflected just within the active optical fiber. Thus, optical amplification at a higher excitation efficiency can be realized.
Preferably, the active optical fiber further includes, at a surface of cladding of the active optical fiber, a spontaneous emission light reflection layer for reflecting the spontaneous emission light. Thus, it becomes possible to confine ASE directing into any non-propagation modes, within the active optical fiber.
Further, an optical amplifying method is provided and which includes (a) generating an excitation light at a predetermined wavelength band; (b) supplying the excitation light to an active optical fiber doped with a rare earth element; (c) reflecting a spontaneous emission light generated from the rare earth element excited by the excitation light and having a wavelength band different from that of the excitation light, and (d) propagating, within the active optical fiber, a signal light at a wavelength band different from the wavelength band of the excitation light and different from the wavelength band of the spontaneous emission light, such that the signal light is amplified within the active optical fiber.